The Mitten Crab is not only a pest but a predator, it will eat crawdads. And it multiplys twice as fast. There in the Delta as well. I copied and pasted some info. from the DGF website.


Larger crabs (wider than 1.5 to 2 inches across the back) have patches of hair on their claws, which look like "mittens." Smaller crabs do not have these patches of hair, but any crab caught in freshwater is probably a mitten crab. Note: There is mitten crab identification information on the DFG’s web page at www.delta.dfg.ca.gov


The mitten crab burrows we have found are no more than 10 to 12 inches deep, which is not very deep when compared to ground squirrel, gopher, and muskrat burrows. However, a large number of mitten crab burrows could accelerate bank and levee erosion rates (although this has yet to be proven). To date, mitten crab burrows have been confirmed only in tidal areas, and have not been found upstream of the delta.


The ecological impact of a large mitten crab population is the least understood of all the potential impacts. Although juveniles primarily consume vegetation, they do prey upon animals, especially invertebrates, as they grow. A large population of mitten crabs could reduce populations of invertebrates, including clams, worms, and other smaller crustaceans through predation, altering the structure of the estuary’s fresh and brackish water benthic invertebrate communities. The mitten crab may also have profound effects on other species through competition. In South San Francisco Bay creeks, the mitten crab and the introduced red swamp crayfish co-occur, overlapping in dietary and habitat preferences. In the delta, the mitten crab may reduce abundance and growth rates of the introduced signal crayfish, which supports a commercial fishery.